Antennas can be placed in arrays to improve directionality or achieve other desired receiving or transmitting characteristics. Many types of arrays have been studied and constructed: the line array; the square lattice planar array; the hexagonal lattice planar array; the ring array; and even random planar arrays. Volumetric (three-dimensional) arrays are also possible and, of course, have three-dimensional frequency response characteristics.
The directionality characteristics of certain antenna are well known. It is known that specific arrays of antennas can be used to increase a response in certain desired directions while suppressing responses from other directions. An antenna array having high directivity will have a minimum of undesired side-lobes and grating lobes. Side-lobes are generally considered to be an undesirable consequence of forming beams through certain antenna arrays. Grating lobes are a type of side-lobe that replicate a maximum response of an array. If not known or accounted for, grating lobes can provide misleading information as to the direction of a received signal. Side-lobe and grating lobe suppression is often attempted through selective weighting of certain antenna inputs of an antenna array. This suppression is also attempted through geometric means, such as by particularly spacing and arranging array sensors with respect to each other.
Many types of square lattice planar arrays are known to the art. Shown in FIGS. 1-4 are four examples of these. Diamond symbols are used to indicate sensor location in these figures and those that follow. The array spacing is denoted by the distance a. In these arrays, as well as the arrays to be described further in this description, terminology taken from crystallography is used to describe certain array features. A source of such terminology is: Transformation Geometry: An Introduction to Symmetry, by Martin, George Edward, Springer-Verlag, New York, 1982.
Using this descriptive language, the square lattice planar array has four-fold rotational symmetry and also has four mirror symmetry planes. Arrays of various orders can be constructed. In general, an nth order square array contains (2n+1)xc3x97(2n+1) points. An array of order zero is a single point. An array of order one has nine points arranged in a 3xc3x973 pattern as shown in FIG. 1. FIG. 2 shows a second order square array containing 25 points. FIG. 3 shows a third order square array containing 49 points. FIG. 4 is an array of order four.
Hexagonal arrays are depicted in FIGS. 5-9, wherein diamonds again depict sensor position. Array spacing is again a. The hexagonal array has six-fold rotational symmetry and also has six mirror symmetry planes. Arrays of various orders can be constructed. In general, an nth order hexagonal array contains 1+3n(n+1) points. An array of order zero is a single point. Referring to FIG. 5, an array of order one has six points equally spaced at a distance a from a central point. The total number of points in the array of order one is seven. FIG. 6 shows an array of order two having twelve additional points for a total of 19 points. FIG. 7 shows a third order hexagonal array containing a total of 37 points. FIGS. 8 and 9 show respectively fourth and fifth order hexagonal arrays.
A ring array consists of N elements equally spaced on the circumference of a circle. The array has N-fold rotational symmetry and N mirror symmetry elements. FIG. 10 shows a configuration of a ring planar array containing six elements. FIG. 11 illustrates a ring array with 18 elements. Ring arrays have proven useful in direction finding applications.
While a great deal of research has been conducted in the field of planar arrays, there is still a need for a planar array configuration that has enhanced directivity and minimal undesired grating and side-lobes.
A planar sensor array described herein as a spiral lattice planar array is comprised of a plurality of sets of sensor elements wherein for each set of the sensor elements an element is disposed at a vertex of an equilateral non-equiangular pentagon. One embodiment includes a plurality of sets of the pentagon arranged elements in an annular array configuration having a centrally located open center defined by the annular array. Another embodiment includes a plurality of sets of the pentagon arranged elements in a core configuration. The core configuration can be disposed within the open center of the annular array configuration. All sensor elements are confined to a single plane. The sensor elements can be equally weighted or may be weighted to provide side-lobe adjustment.
An object of this invention is to provide a sensor array that has enhanced directivity.
A further object of this invention is to provide a sensor array that minimizes undesired side-lobes.
Still a further object of the invention is to provide a sensor array that minimizes undesired grating lobes.
Still yet another object of this invention is to provide a planar sensor array that utilizes array geometry to minimize undesired side-lobes and grating lobes and that provides desired directivity.
Other objects, advantages and new features of the invention will become apparent from the following detailed description when considered in conjunction with the accompanied drawings.